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In this study, we demonstrate solution-processed memristor devices using a CdSe/ZnS colloidal quantum dot (CQD)/poly(methyl methacrylate) (PMMA) composite and their electrical characteristics were investigated. Particularly, to obtain stable memristive characteristics with a large current switching ratio, the concentration of CdSe/ZnS QDs in the PMMA matrix was optimized. It was found that with the CdSe/ZnS QD concentration of 1 wt%, the memristor device exhibited a high current switching ratio of ~10⁴ and a retention time over 10⁴ s, owing to the efficient charge trapping and de-trapping during the set and reset processes, respectively. In addition, we investigated the operational stability of the device by carrying out the cyclic endurance test and it was found that the memristor device showed stable switching behavior up to 400 cycles. Furthermore, by analyzing the conduction behavior of the memristor device, we have deduced the possible mechanisms for the degradation of the switching characteristics over long switching cycles. Specifically, it was observed that the dominant conduction mechanism changed from trap-free space charge-limited current conduction to trap charge-limited current conduction, indicating the creation of additional trap states during the repeated operation, disturbing the memristive operation.